1. Field of the Invention
The invention relates to a method for operating an exhaust-gas aftertreatment system in internal combustion engines operated with an excess of aft, such as diesel engines and gasoline engines with direct injection.
2. Description of Prior Art
Aside from solid particles, nitrogen oxides are among the limited exhaust-gas components which are generated during combustion processes and the permitted emissions of which are being progressively lowered. Various methods are currently used to minimize the exhaust-gas components in internal combustion engines operated in motor vehicles. The reduction of the nitrogen oxides is realized usually with the aid of catalytic converters; in oxygen-rich exhaust gas, a reducing agent is additionally necessary in order to increase the selectivity and the NOx conversion. These methods have become known under the collective term SCR methods, wherein SCR stands for “selective catalytic reduction”. These methods have been used for many years in the field of power plants and recently &so for internal combustion engines. A detailed explanation of such methods emerges from DE 34 28 232 A1. In practical applications, ammonia or compounds which split to form ammonia, such as urea or ammonium formate, in solid form or in solution, are used as reducing agent. For the conversion from one mole of nitrogen monoxide, one mole of ammonia is required, wherein the nitrogen oxides react with the NH3 deposited on the catalytic converter in accordance with the following equation:4NO+4NH3+O24N2+6H2O  (1).
The ratio between NH3 and NOx is referred to as the feed ratio α.α=NH3/NOx 
In the case of an ideal catalytic converter, this means that, for a feed ratio of α=1, all of the nitrogen oxides are reduced, that is to say 100% NOx conversion is attained, because the following applies for the NOx conversion XNOx:
      X    NOx    =                    C                  NOx          ,          0                    -              C        NOx                    C              NOx        ,        0            where: cNOx,0: NOx untreated emissions [ppm]                cNOx: NOx emissions downstream of catalytic converter [ppm]        
If a platinum-containing NO oxidation catalytic converter for forming NO2 is positioned upstream of the SCR catalytic converters2NO+O22NO2  (2)then the SCR reaction can be accelerated considerably, and low-temperature activity can be markedly increased.NO+2NH3+NO22N2+3H2O  (3)
In the case of internal combustion engines operated in vehicles, the nitrogen oxide reduction with the aid of the SCR method is difficult because fluctuating operating conditions prevail there, which makes the quantitative metering of reducing agent difficult. It is duly sought, on the one hand, to attain as high as possible a conversion of nitrogen oxides; it must on the other hand be ensured that no emissions of unconsumed ammonia occur. To remedy this, use is often made of an ammonia blocking catalytic converter which is positioned downstream of the SCR catalytic converter and which converts excess ammonia into nitrogen and water vapour.
To minimize fine particle emissions, particle filters are used both in the power plant field and also in vehicles. In particle filters, the diameter of the filter ducts lies in the range of the diameters of the particles. Owing to this fact, particle filters are at risk of becoming blocked, which increases the exhaust-gas back pressure and reduces engine power. An arrangement and a method with a particle filter emerge from EP 0 341 832 A2. The abovementioned arrangements and the abovementioned method are characterized in that the oxidation catalytic converter—usually a catalytic converter with platinum as active material—arranged upstream of the particle filter oxidizes the nitrogen monoxide in the exhaust gas, with the aid of the residual oxygen likewise present in the exhaust gas, to form nitrogen dioxide, which in turn is converted in the particle filter with the carbon particles to form CO, CO2, N2 and NO. A continuous removal of the accumulated fine particles is realized in this way; regeneration cycles such as must be performed in a cumbersome manner in the case of other arrangements are thereby dispensed with.2NO2+C2NO+CO2  (4)2NO2+C2NO+CO  (5)2C+2NO2N2+2CO2  (6)
If complete oxidation of the carbon stored in the particle filter is not attained with the aid of NO2, the carbon fraction and thus the exhaust-gas back pressure progressively increase. If a critical mass is reached, an uncontrolled ignition of the carbon may occur at high exhaust-gas temperatures, which carbon then burns abruptly with oxygen.C+O2CO2  (7)
This leads to a temperature rise to up to 1000° C.
The simultaneous use both of arrangements for reducing nitrogen oxide emissions and also arrangements for reducing fine particle emissions is necessary in order to comply with the future exhaust-gas regulations. Various arrangements and methods for this purpose have already become known.
DE 103 48 799 A1 describes an arrangement composed of an oxidation catalytic converter, an SCR catalytic converter arranged downstream of the oxidation catalytic converter in the exhaust-gas flow, and a particle filter arranged, in turn, downstream of the SCR catalytic converter in the exhaust-gas flow. The supply of the reducing agent for the selective catalytic reaction that takes place in the SCR catalytic converter takes place directly upstream of the SCR catalytic converter by means of a urea injection device that is controlled as a function of operating parameters of the internal combustion engine. A disadvantage of this arrangement is that the nitrogen dioxide generated in the oxidation catalytic converter is substantially completely consumed by the selective catalytic reduction in the SCR catalytic converter, that is to say is not available for the conversion of the fine particles that have accumulated in the downstream particle filter. The regeneration of the particle filter must therefore be effected in a cumbersome manner by cyclic heating of the exhaust-gas flow by virtue of the exhaust-gas flow being enriched with unburned hydrocarbons and these subsequently being catalytically oxidized. This is realized either through enrichment of the combustion mixture or the injection of fuel upstream of the particle filter. Such an arrangement for the regeneration of the particle filter is firstly cumbersome and therefore expensive, and, secondly, the cyclic regeneration of the particle filter situated at the end of the arrangement generates new pollutants, such as carbon monoxide, which can be removed from the exhaust gas again only with difficulty.
A further combination of a particle filter and an arrangement for selective catalytic reduction has become known from EP 1 054 722 A1. The arrangement described therein is composed of an oxidation catalytic converter, which is arranged in the exhaust-gas flow and which increases the fraction of nitrogen dioxide in the exhaust gas, a fine particle filter arranged downstream, a reservoir for the reducing fluid, an injection device for the reducing fluid, the injection device being arranged downstream of the fine particle filter, and an SCR catalytic converter arranged downstream of the injection device in the exhaust-gas flow. The arrangement described above duly permits a continuous conversion of the soot-type fine particles that have accumulated in the fine particle filter with the aid of the nitrogen dioxide generated in the oxidation catalytic converter, but has another major disadvantage. The particle filter causes cooling of the exhaust gas, such that if for example the commercially available reducing fluid named AdBlue is used, it is the case in particular after starting of the internal combustion engine or during operation of the internal combustion engine in the lower power range that the exhaust-gas temperature is too low to generate ammonia from the 33% aqueous urea solution without the formation of problematic by-products.
In conjunction with the breakdown of urea ((NH2)2CO) to form ammonia (NH3), it is known that, under optimum conditions (temperatures above 350° C.), this takes place in two stages; firstly, the thermolysis, that is to say the thermal breakdown of urea, takes place as per the equation(NH2)2CONH3+HNCO  (8).
Subsequently, the hydrolysis, that is to say the catalytic breakdown of isocyanic add (HNCO) to form ammonia (NH3) and carbon dioxide (CO2), takes place as per the equationHNCO+H2ONH3+CO2  (9).
Since, if AdBlue is used, the reducing agent is present in a form dissolved in water, the water must evaporate before and during the actual thermolysis and hydrolysis.
If the temperatures prevailing during the above reactions (7) and (8) are below 350° C. or if heating takes place only slowly, it is known from DE 40 38 054 A1 that primarily solid, non-melting cyanuric add is generated by trimerization of the isocyanic add formed in equation (7), as per3H CO<350° C. . . . . . . >350° C.(HNCO)3  (10),which cyanuric add leads to blockage of the downstream SCR catalytic converter. This may be remedied, as stated in the cited DE 40 38 054, by virtue of the exhaust-gas flow which is laden with reducing agent being conducted across a hydrolysis catalytic converter. The exhaust-gas temperature above which quantitative hydrolysis becomes possible can thus be pushed to 160° C. The construction and composition of a corresponding catalytic converter is described in the cited publication, as is the construction and function of an SCR catalytic converter system equipped with a hydrolysis catalytic converter. Such an additional hydrolysis catalytic converter however additionally increases the cost of the arrangement for exhaust-gas aftertreatment.
An exhaust-gas aftertreatment arrangement composed of an oxidation catalytic converter and a downstream particle filter which is combined with an SCR catalytic converter to form a structural unit such that the particle filter is coated with an SCR-active compound or is composed of such a compound or has embedded SCR-active centres, is known from DE 103 23 607. For the introduction of the reducing agent, it is provided in this arrangement that a supply means for the reducing agent, preferably an aqueous urea solution, is provided downstream of the oxidation catalytic converter and upstream of the particle filter. Such an arrangement duly has the advantage of a small structural size and a low heat capacity, but, as tests have shown, has the disadvantage that, as a result of the ammonia storage property of SCR-active catalyst materials, a significant excess of ammonia can occur in those regions of the arrangement which are directly adjacent to the accumulated particles, such that the reaction as per equation (3) competes directly in a very limited space with the reaction as per equations (4) to (6). Therefore, fewer of the accumulated carbon particles are converted than would be necessary in order to avoid an increase of the exhaust-gas back pressure and the risk of an uncontrolled burn-off of the accumulated carbon particles as per equation (7) in the event of a critical particle mass being reached. The first results in a power decrease if the exhaust-gas aftertreatment arrangement is installed in the exhaust tract of an internal combustion engine, and the second results in possible destruction of the entire arrangement.
A further arrangement for exhaust-gas aftertreatment emerges from EP 2014348. This document specifies a method for the reduction of nitrogen oxides, which method comprises the following steps;
a) means for providing NO2 in the exhaust gas which contains NOx and soot, wherein the means are engine-internal measures, and/or at least some of the exhaust gas which contains NOx and soot is brought into contact with an oxidation catalytic converter which increases the NO2/NO, ratio;
b) an injection module which is designed to inject a predetermined amount of ammonia at least into some of the exhaust gas which contains NOx and soot, the ammonia being in the form of either pure ammonia or a precursor compound for ammonia such as urea, a liquid solution of urea, ammonium carbamate, isocyanic acid, cyanuric acid, methaneamide, etc. or combinations of these, wherein the injection module is arranged downstream of or parallel to the oxidation catalytic converter in the flow direction of the exhaust gas which contains NOx and soot, and
c) a device for the filtering/separation of soot, that is to say carbon particles, wherein the NOx contained in the exhaust gas and the soot are intended to make contact with the soot that has accumulated in the device for the filtering/separation of soot, that is to say the carbon particles, with the ammonia-enriched exhaust gas which contains NOx and soot, whereby a selective catalytic reduction of at least some of the NOx molecules with the ammonia to form nitrogen and water is initiated.